There is a technique to improve visibility at the time when the vehicle moves backward. In the technique, by displaying an image behind a vehicle taken by an on-vehicle camera on an on-vehicle monitor, a state just close to a place behind a vehicle which is a blind spot for the driver is observed as an image displayed on the on-vehicle monitor.
For displaying such an image of the on-vehicle camera, there is a technique for calibrating a state of attachment of the on-vehicle camera to the vehicle. For the calibration, a calibration index is provided behind the vehicle, and the driver views an image of the calibration index displayed on the on-vehicle monitor, and thereby simultaneously adjusts the state of attachment of the on-vehicle camera so that the image of the calibration index can be appropriately displayed.
In addition, there is a technique to appropriately calibrate an image displayed on the on-vehicle monitor by performing certain computation on the image obtained using the on-vehicle camera based on the image of the calibration index (Patent Document 1).
Moreover, there is another technique to be performed in the following manner. Multiple on-vehicle cameras take images of places all around the vehicle, multiple images obtained by the on-vehicle cameras are transformed into such images (bird's-eye view images) as being looked down from directly above the vehicle, and mapping is performed while relative locations of the images are adjusted. Then, a single viewpoint-transformed composite image is obtained (see Patent Document 2).
The above cases require accurate positioning of two adjacent images relative to each other, thus requiring highly accurate calibration.